As the cartridge is a floating source, there is no possibility of creating a ground loop unless the turntable wiring is faulty
![Index pointing up :point_up: ☝️](https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/261d.png)
Which is unfortunately rather common with certain entry-level (a.k.a. "cheapo") turntables. For example, the original
AT-LP120-USB
phono cartridge signal path has a non-bypassable L-C filter circuit always in place when set to "Phono" mode (e.g. when supposed to bypass the internal preamp). Additionally, there appears to be a SMD component jumpering all of the cartridge's negative lines as they come out from the tonearm and into the circuitboard inside the chassis. Audio-Technica was so "
kind" as to include this pre-soldered ground loop path which is permanently installed for the user.
Another example would be this
Udreamer 009
turntable which is currently popular on Amazon. Testing this one revealed multiple issues, including a built-in ground loop when in bypass mode. Another hilariously bad issue with this one: The
"Line" / "Phono"
switch was actually labeled in reverse! So, when switched to "Phono", actually a Line-level signal was output on the RCA jacks, except without any bass, and with very harsh high frequency range. It seemed like they forgot to apply the RIAA curve when in internal preamp (Line-level) mode (even if it was labeled backwards). There was no continuity between RCA pin and shield with the headshell pins when the switch was set to "Phono" mode and the level of the signal was at Line-level instead. When switched to "Line" mode, as labeled, then the headshell pins were connected to the RCA jacks (as it would be for Phono / "bypass" mode). However, both negative (
L-
,
R-
) lines were shorted to each other AND the chassis ground lug. So yet another built-in ground loop path when chassis lug & an external preamp ground lug are connected.
A final example, is even
mentioned on the Pro-ject site regarding the use of certain MM cartridges:
Why should I avoid using a moving magnet cartridge?
Moving magnet cartridges are also inherently balanced transducers but the balanced signal ends at the cartridge output pins. For noise suppression in unbalanced systems, manufacturers usually tie the right channel grounding conductor to the cartridge’s chassis. This renders the signal unbalanced henceforth and consequently, balanced connection post-cartridge will introduce grounding issues that’ll generate
more noise, but just in the unbalanced right channel. Here’s that graph again to show you just what we mean (hone in on Graph 2!)…
Graph 1: typical moving magnet (MM) phono cartridge connected with standard RCA/ground interconnects (noise identical in both channels)
Graph 2: typical MM cartridge connected with Mini XLR or XLR interconnects (higher noise level of -55dB in right channel vs. -61dB on left channel – not advisable!)
Graph 3: typical moving coil (MC) phono cartridge connected with standard RCA/ground interconnects (significantly quieter than MM but significantly noisier than balanced as shown in Graph 4)
Graph 4: typical MC cartridge connect
This issue may not affect all moving magnet type cartridges, and some include a removable ground clip (usually on the R- line, but sometimes L-).
Shure has this page describing how to remove the jumper clip for their cartridges.
So, as with most things of sufficient complexity... "The devil's in the details"... and there's a lot of details to get wrong! (Also, the "
great" thing about standards is there are so many to choose from ) It seems Pro-ject is recommending to use MC cartridges as a guaranteed way to achieve a truly floating & balanced signal path from the headshell. Presumably this is because the moving coil design is less likely to allow for manufacturers of the cartridge to short one of the
- signal
pins.